The present invention is related to the manufacture of III-V light emitting and laser diodes, particularly towards improving the characteristics of the electrical contact to the p-type portion of the diode.
Gallium nitride (GaN) compounds have wavelength emissions in the entire visible spectrum as well as part of the UV. FIG. 1 illustrates a typical GaN-based light emitting diode (LED). Currently, most GaN-based LEDs are epitaxially grown on a sapphire or silicon carbide (SiC) substrate. A double hetero-structure that includes a nucleation layer, n-type layer, active region, p-type AlGaN layer, and a p-type layer of GaN is formed on the substrate. In general, the ability to fabricate ohmic contacts to the p-type layer is desirable for the realization of reliable light emitting diodes and laser diodes. Ohmic contacts to p-type GaN are difficult to achieve because the attainable hole concentration is limited for Mg-doped III-nitride based semiconductors. In addition, many light-emitting diodes and vertical cavity surface-emitting laser diodes use thin, transparent metal contacts. The choice of metals is limited and metal layers need to be thin, e.g.  less than 15 nm, to reduce light absorption. Because there is poor lateral current spreading in p-type GaN, the metal layers typically cover nearly the entire device area.
P-type conductivity for GaN is achieved by doping with Mg, which substitutes for gallium in the GaN lattice and acts as an acceptor (MgGa). MgGa introduces a relatively deep acceptor level into the band gap of GaN. As a consequence, only xcx9c1% of the incorporated Mg acceptors are ionized at room temperature. To illustrate, a Mg concentration ([Mg]) of xcx9c5e19 cmxe2x88x923 is needed to achieve a room temperature hole concentration of xcx9c5e17 cmxe2x88x923. Further, Mg-doped GaN requires a post-growth activation process to activate the p-type dopants. The post-growth activation process may be, for example, thermal annealing, low-energy electron-beam irradiation, or microwave exposure. For conductivity-optimized Mg-doped GaN layers, [Mg] less than 5e19 cmxe2x88x923, the acceptor concentration (NA) is about equal to the atomic Mg concentration and the resistivity can be around 1 xcexa9cm or less. These layers may be referred to as xe2x80x9cp-type conductive layersxe2x80x9d. Increasing the Mg content beyond approximately 5e19 cmxe2x88x923 does not translate to higher acceptor concentration. Typically, a reduction of NA is observed when the [Mg] exceeds a certain maximum concentration and the layer becomes resistive.
P-type layers of a III-nitride-based light-emitting device are optimized for formation of an Ohmic contact with metals. In some embodiments, a p-type transition layer is formed between a p-type conductivity layer and the metal contact. The p-type transition layer may be a GaN layer with a resisitivity greater than 7 ohm-centimeters, a III-nitride layer, a III-nitride layer with added As or P, or a superlattice with alternating highly doped or elemental dopant sublayers and lightly doped or undoped sublayers.
In some embodiments, the p-type layer is continuous with varying levels of dopant. The concentration of dopant in the region of the p-type layer adjacent to the p-contact is greater than the concentration of dopant in the region of the p-type layer adjacent to the active region. The p-type layer may also have a varying composition, for example of Al or In or both.